In order to produce injection mouldings of different shapes and sizes, the use of a mixture of polypropylene and sawdust prepared before injection is known, the proportion of sawdust being generally between 10 and 50% by weight.
Heat-moulding of a material of this type can be done in all conventional injection devices designed for moulding thermoplastic materials. The sawdust and polypropylene mixture may be introduced into the cylinder (or cylinders) or heated chamber (or chambers) of a standard injection machine where the thermoplastic resin is softened; a compression organ, such as a piston or endless screw, then injects the mixture of melted thermoplastic resin and sawdust, through an injection head at the end of the cylinder, into a mould.
The temperature of the front part of the cylinder is usually set to about 160.degree. C. and that of the rear part to about 180.degree. C., these values may vary within a range of +/-15.degree. C. depending on the shape and size of the moulding to be made, the quantity of material required and other moulding parameters such as the injection pressure, the proportion of sawdust in the mixture, etc. Thus, for a set moulding to be made, there is a temperature t.sub.1 for the rear part of the cylinder of the injection machine and a temperature t.sub.2 for the front part of the said cylinder--t.sub.1 being lower than t.sub.2 --which corresponds to an optimum injection moulding operation, giving the moulding a uniform appearance and the physical characteristics usually achieved with this type of material. Moreover, it is also known that the material injected into the mould must never exceed a temperature of 210.degree. C. which corresponds to a threshold of decomposition of the wood. If this temperature is reached or exceeded, the material thus heated is degassed and expands so that the moulding obtained is totally unusable.
In French Patent 2 413 205, it has already been stated that by increasing the optimum temperatures at the front and rear parts of the injection machine chamber by a value .DELTA.t within 5.degree. and 15.degree. C. without exceeding the threshold of 210.degree. C. of decomposition of the wood, a moulding of a darker colour is obtained, the wall of which has marbling giving it the appearance of wood. This decorative feature is obtained without distorting the moulding in any way and is perfectly reproduceable. It is assumed that the increase in the optimum temperatures at the front and rear parts of the chamber transforms the sawdust contained in the mixture but the heating temperatures are insufficient to cause decomposition, degassing and expansion of the injected material. Mouldings with the appearance of wood are thus obtained.
Moreover, Patent JP-A 63207610 describes the manufacture of a product with the appearance of marble by preparing a pre-impregnated sheet containing fibres and a mineral filler and a thermohardenable resin by laminating the impregnated sheet and heat-hardening it. Belgian Patent BE-A 385 202 concerns a process for manufacturing artificial sandstone in which a thermohardenable resin in a dry state or in solution with fibrous materials is mixed and the mixture compressed in heated moulds under high pressure.